Consider making throw and throwIO strict
It's possible for code to throw an exception that itself throws an imprecise exception. Such an exception is tricky to catch. For example:
import Control.Exception strange = throwIO (undefined :: SomeException) `catch` \ex -> case () of _ | Just _ <- (fromException ex :: Maybe IOError) -> print "IOError" | otherwise -> print "Something else"
You might think that this would catch the exception and print "Something else", but in fact it does not. If others think this is as surprising as I do, perhaps we should make
throw strict, so an exception will never itself be bottom. Using
throwIO' !e = throwIO e
in the code above instead of
throwIO allows the exception to be caught.